The invention relates to a radar sensor device for detecting the distance and/or the speed of an object relative to the sensor device.
Radar technology is particularly suitable for use in motor vehicles and in industrial applications for a contactless detection of object data such as range, speed, state or presence. The functionality, measuring accuracy and production costs of radar sensors depend in this case substantially on the modulation methods applied and on the associated radar signal processing. The respective constraints of an application determine the configuration of the components of a radar sensor device and thus, for example, the complexity or simplicity of the electronics used in this case.
A contactless measurement of distance and speed with the aid of radar has been practiced for many years, mainly in military technology. In this connection, two different standard modulation methods are known for distance measurement, specifically pulse modulation and frequency modulation.
In the pulse travel time method, a short radar pulse is emitted in the direction of a measurement object and is received again as a pulse reflected from an object after a specific travel time. The travel time of the radar pulse is directly proportional to the distance from the measurement object.
In the case of the frequency method, a frequency-modulated radar signal is emitted and is received with a phase shift or frequency shift. The measured phase or frequency difference, which is typically in the kHz region, is proportional to the object distance. A requirement for this is a temporally linear frequency modulation.
Theoretically, the measured values obtained by the pulse travel time method on the one hand and the frequency method on the other hand are equivalent. In practice, however, the methods have specific advantages and disadvantages with reference to the parameters for the sensor measuring accuracy and the achievable structural resolution which are of relevance in practice. These parameters are first and foremost the modulation bandwidth and the radar transmitted/received power. For these parameters, which determine the structural resolution and range, there are radio approval regulations and technically and economically relevant constraints or boundary conditions, in particular with regard to the functioning and the outlay on circuitry. In this context there may be mentioned, for example, the frequency bands, which can be used for short-range monitoring with the aid of radar sensors, from 24.0 to 24.25, from 61.0 to 61.5 and from 76.0 to 77.0 GHz, to which the radar parameters mentioned below also relate.
For the application of the measurements of distance and speed of objects in the surroundings of a motor vehicle, a physical structural resolution of  less than 15 cm is required for a measuring range of 0 to 5 meters (short range). A modulation bandwidth of  greater than 1 GHz is required for this purpose. A pulse duration of  less than 1 ns is required in the case of an appropriate pulse method. The generation of the abovementioned radar signals with a duration of a few 100 ps, a bandwidth  greater than 1 GHz and a radar central frequency at, for example, 24 or 77 GHz entails a not inconsiderable technical outlay.
For reasons of costs, known radar sensor devices for industrial applicationsxe2x80x94for example a filling level radarxe2x80x94and traffic applicationsxe2x80x94for example a motor vehicle distance radarxe2x80x94mainly use frequency-modulated methods, since in this case a flexible and very precise digital signal processing is possible. Furthermore, a high range of up to approximately 100 m is achieved even for objects of low reflectivity.
U.S. Pat. No. 5,325,097 discloses a radar system for road vehicles which distinguishes between dangerous and non-dangerous target objects inside a predetermined zone. This system uses a pair of frequency-modulated continuous wave radar cycles and a single continuous wave cycle in generating radar signals for measuring the target region and the apparent target speed. Measured values from the FM/CW (frequency modulation/continuous wave) and CW (continuous wave) cycles are used with predetermined values for distinguishing between dangerous and non-dangerous target objects. With reference to its configuration, the system known from U.S. Pat. No. 5,325,097 has a frequency-detunable oscillator, which can be driven electronically through the use of a modulation function, for generating a transmitted signal, a transmitting antenna for emitting the transmitted signal, a receiving antenna for receiving the radar received signal reflected by the target, a demodulation unit for forming a demodulated received measuring signal, and a control unit in the form of a digital processor, which serves to control the sensor device and, above all, to control the modulator, and to evaluate the received signal.
German Patent No. DE 38 30 992 C2 discloses a radar altimeter which is configured using the FM-CW principle. The power of the transmission path of the radar equipment used in this case can be controlled by a variable attenuator connected upstream of a power amplifier, specifically as a function of the altitude and, in particular flight altitude of the aircraft equipped with the radar altimeter. It is clear that the radar equipment is always to be operated with as little radar energy as possible, in order to keep the detectability by extraneous sensors as low as possible, which is important, in particular, in the case of military applications.
Despite their practical applicability, for various reasons the known radar sensor devices explained above and based on a pulse or frequency method are unsuitable for the detection tasks at short range, which are of interest in conjunction with the present invention. Specifically, in the case of the present invention, strongly differing measuring requirements, such as near and remote targets, low and high object speeds and different object reflectivities occur. These measuring requirements must be fulfilled simultaneously.
Published, Non-Prosecuted German Patent Application No. DE 195 38 309 A discloses a radar sensor device for measuring distances and relative speeds between a vehicle and one or more obstacles, which has an oscillator for generating a transmitted signal whose frequency can be varied through the use of a control modulation voltage. Radar signals are emitted via a transceiver antenna, and radar received signals reflected by an object to be detected are received. The received signal is demodulated by a single-channel mixer and evaluated in a signal processing device.
From U.S. Pat. No. 5, 287, 111 it is further known in principle to vary the detection range of radar sensors by varying the transmitter power.
It is accordingly an object of the invention to provide a radar sensor device which overcomes the above-mentioned disadvantages of the heretofore-known devices of this general type and which has a high flexibility and, in particular, effective applicability at short range in conjunction with an acceptable outlay on circuitry.
With the foregoing and other objects in view there is provided, in accordance with the invention, a radar sensor device for detecting a distance and/or a speed of an object relative to the radar sensor device, including:
a frequency-detunable oscillator operating with a given frequency and a given phase for generating a transmitted signal;
a transmitting antenna, connected to the frequency-detunable oscillator, for emitting a radar signal based on the transmitted signal;
a receiving antenna for receiving a radar received signal reflected by an object to be detected;
a demodulation unit, connected to the frequency-detunable oscillator and to the receiving antenna, for forming a demodulated received measuring signal from the transmitted signal and the radar received signal;
a control unit, connected to the frequency-detunable oscillator, for electronically driving the frequency-detunable oscillator with a modulation function and for evaluating the demodulated received measuring signal;
a power switch connected between the frequency-detunable oscillator and the transmitting antenna, the control unit driving the power switch with a power control function for varying a transmitter power for the transmitting antenna;
a reference unit assigned to the frequency-detunable oscillator, the reference unit generating a reference signal for monitoring the given frequency and the given phase of the frequency-detunable oscillator; and
the control unit setting the modulation function and the power control function in a variable manner, and controlling the frequency-detunable oscillator and the power switch alternatingly in at least two different, mutually overlapping operating modes tuned to different measuring ranges.
Accordingly, a combined and flexible modulation method is used in the case of the radar sensor device according to the invention, the sensor device can be operated by a variable setting, performed by a control unit, of a modulation and power control function in an alternating fashion in at least two different, mutually overlapping operating modes. In this case, an electronically drivable, frequency-detunable oscillator is driven with the aid of the modulation function in order to generate a transmitted signal for a transmitting antenna. The power control function controls a power switch which is connected between the oscillator and the transmitting antenna for the purpose of varying the transmitter power. The radar received signal generated by the receiving antenna is combined together with the transmitted signal in a demodulation unit to form a demodulated received measuring signal. The latter can be evaluated by the control unit S mentioned above. The control unit serves simultaneously to control the entire sensor device. It is possible for the control unit to monitor the frequency and phase of the oscillator through the use of a reference unit assigned to the oscillator.
Due to this basic configuration of the radar sensor device, the radar parameters of xe2x80x9cbandwidthxe2x80x9d and xe2x80x9cpowerxe2x80x9d can be adaptively set through the use of the control unit in the two overlapping operating modes by appropriately driving the power switch and the oscillator. For a distance measurement at short range, the operation is performed through the use of a frequency-modulated operating mode of reduced transmitter power but, in return, of higher bandwidth on which there is superimposed for speed measurement both at short range and at long range an operating mode with fixed frequency operation and normal output power. It is to be noted in this case that the transmitter power of the sensor device can be varied in accordance with the currently obtaining reflectivity of the detected objects due to the fact that the power switch can be driven variably. This avoids overdriving of the receiving elements. The dynamic range of the sensor device can therefore be set adaptively in an optimum fashion with reference to a respective situation.
In accordance with another feature of the invention, the control unit controls the frequency-detunable oscillator and the power switch for generating, in an alternating clocked sequence, a monofrequency transmitted signal in a CW operation mode, and a frequency-modulated transmitted signal in an FM operation mode.
In accordance with yet another feature of the invention, the control unit adaptively drives the frequency-detunable oscillator and the power switch by setting the modulation function and the power control function such that, in an FM operation mode, the transmitter power is reduced to a low power level and the given frequency is modulated in a wide bandwidth for detecting a distance of an object at short range, and such that, in a CW operation mode, the transmitter power is not reduced and remains at a high power level and the given frequency is a fixed frequency associated with a narrow bandwidth for detecting a speed of an object at least at long range, the high power level being higher than the low power level and the wide bandwidth being wider than the narrow bandwidth.
In accordance with another feature of the invention, the control unit selectively sets the modulation function as a amp-shaped modulation function in the FM operation mode, and as a constant modulation function in the CW operation mode.
In accordance with a further feature of the invention, the control unit sets the power control function as an amplitude switching function for switching between two radar power levels.
In accordance with another feature of the invention, the control unit sets the amplitude switching function with a given switching rate and the modulation function with a given modulation rate such that the given switching rate is higher than the given modulation rate.
In accordance with yet another feature of the invention, the frequency-detunable oscillator and the power switch define a transmission path; the transmitting antenna is separate from the receiving antenna; the demodulation unit includes a power divider and a receiving mixer; the power divider is provided in the transmission path and separates off a portion of the transmitted signal for supplying a separated portion of the transmitted signal to the receiving mixer; and the receiving mixer is assigned to the receiving antenna and receives the separated portion of the transmitted signal and the radar received signal for forming the demodulated received measuring signal.
In accordance with another feature of the invention, the frequency-detunable oscillator and the power switch define a transmission path; the transmitting antenna and the receiving antenna form a combined transmitting/receiving antenna; and the demodulation unit is a bidirectional mixer provided in the transmission path.
In accordance with a further feature of the invention, the power switch is a switchable high-frequency amplifier, in particular a switchable high-frequency transistor.
In accordance with another feature of the invention, the radar sensor device includes a sample-and-hold unit connected downstream of the demodulation unit.
In accordance with another feature of the invention, the control unit sets the power control function as a clocked power switching function; and the sample-and-hold unit operates time-synchronized with respect to the clocked power switching function.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a radar sensor device, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.